1. Field of the Invention
This invention relates to improvements in or relating to a mass spectrometer and is more particularly concerned with a form of mass spectrometer which utilizes trapping of the ions to be analyzed.
2. Description of the Prior Art
Molecular or atomic weight of a substance is a useful characteristic which, if detected, can enable the substance to be identified. A mass spectrometer is a measuring instrument which can determine the molecular weight of a substance or other molecule introduced into it for analysis. Mass Spectrometers operate in a number of different ways, however the present invention is concerned particularly with mass spectrometers in which ions are trapped or confined within a particular region of space for analysis purposes. Known types of mass spectrometers of this type are the so-called "quadrupole ion trap" spectrometers and "ion cyclotron resonance" spectrometers.
Quadrupole ion trap mass spectrometers currently available use a three-dimensional quadrupole electric field which oscillates at radio frequencies to trap ions. The ions can then be ejected from the field selectively on the basis of mass/charge ratio enabling the device to operate as a mass spectrometer. This form of spectrometer can be produced relatively inexpensively and relatively small in size, making it a popular choice as a mass selective detector for gas chromatographs (GC-MS).
Ion cyclotron resonance (ICR) mass spectrometers currently available use a combination of an electric field and a very strong magnetic field to trap ions. The trapped ions spiral around the magnetic field lines with a frequency related to the mass of the ion. The ions are then excited such that the radii of their spiralling motion increases and as the radii increase the ions are arranged to pass close to a detector plate in which they induce image currents. The measured signal on these detector plates as a function of time is related to the number and frequencies (hence mass) of the ions. Conventional techniques such as Fourier transformation can be applied to the measured signal to obtain the component frequencies of the ions and hence produce a frequency (and hence mass) spectrum. This type of mass spectrometer is able to produce a very high degree of mass resolution.
However, there-are disadvantages associated with the known forms of mass spectrometer described above. For instance, while the quadrupole ion trap mass spectrometer can be constructed small and cheaply, the mass resolution and mass range obtained is not very high unless the analysis is carried out using very slow scanning. While this is adequate for gas chromatograph mass measurement, it limits the applicability to molecular weight molecules of a biochemical nature. Furthermore, with the ion cyclotron resonance mass spectrometer described above, in order to provide the high magnetic field necessary for the spectrometer to work efficiently, it is necessary to provide a super conducting magnet which in itself is very expensive. Furthermore, a super conducting magnet of the type necessary requires, with technology currently available, the use of liquid helium to cool it and as a continuous supply of this is required, it necessarily results in high running costs of the spectrometer due to the relatively high cost of liquid helium.